Mobile data transmission and data services are constantly making progress. With the increasing penetration of such services, different access networks may coexist in parallel. Typically, in relation to mobile communication systems, an access network is represented by a radio access network (RAN) which is based on a certain radio access technology (RAT). While “radio” is a typical medium for mobile communication, other media are intended to be also covered by the principles taught herein. For example, Infrared or Bluetooth® or other media and/or wavelengths of radio are possible to represent the access network. As there has to be a (downward) compatibility between newly developed and pre-existing access networks and/or access networks technologies, terminals often have a capability to communicate based on one or more access networks technologies. Also, when a new access network is developed and launched, the network is not immediately available in the entire country of deployment, but its coverage may be limited to certain areas and be successively expanded over time.
The present invention will herein below be explained with reference to Long Term Evolution (LTE) as one example of a access network or radio access technology (LTE is also known as fourth generation 4G mobile communication) and its successor or improvement which is currently being developed and referred to as 5G (fifth generation mobile communication) as a further access network or radio access technology, but also with reference to predecessors thereof. Though, principles set out herein below are applicable to other scenarios than explained, too. Typically, a mobile communication network consists of an access network establishing the physical transport of data (payload (user) data and control data) and a core network establishing the control functionality for the entire network and the interoperability of the network with other networks, e.g. via gateways. References to specific network entities or nodes and their names are intended as mere example only. Other network node names may apply in different scenarios while still accomplishing the same functionality. Also, the same functionality may be moved to another network entity. Therefore, the principles as taught herein below are not to be understood as being limited to the specific scenario referred to for explanation purposes.
For example, the Evolved Packet System (EPS) is the successor of General Packet Radio System (GPRS). It provides a new radio interface and new packet core network functions for broadband wireless data access. Such EPS core network functions are the Mobility Management Entity (MME), Packet Data Network Gateway (PDN-GW, P-GW) and Serving Gateway (S-GW).
FIG. 1 illustrates the Evolved Packet Core architecture as introduced and defined by 3GPP TS 23.401 v13.0.0.
The entities involved and interfaces there between are defined in that document and reference is made thereto for further details. Acronyms used in the Figure are listed at the end of this specification.
A common packet domain Core Network (CN) is used for both RANs, the Global System for Mobile Communication (GSM) Enhanced Radio Access Network (GERAN) and the Universal Terrestrial Radio Access Network (UTRAN). This common CN provides GPRS services.
It is envisioned that a 5G system will provide the new mobile, low-latency and ultra-reliable services, and some services like Vehicle-To-X (V2X) will be more efficiently provided by 5G system.
A reference to 5G architecture that is envisioned is depicted in FIG. 2.
Acronyms used in the Figure are listed at the end of this specification.
In brief, a terminal such as a 5G NT (network terminal or user equipment) is provided with an internet protocol IP user network interface (IP UNI) and an Ethernet user network interface (ETH UNI) and may communicate via a Uu* interface with an Access Point (AP) in the mobile access network. The entire network has a mobile access part, a networks service part and an application part. Within each of those parts, there exists a control plane and a user (data) plane. The AP is located in both planes.
It is evident that interworking of 5G with the existing RAT technologies like LTE is needed
FIG. 3 schematically illustrates possible inter RAT architecture following traditional concepts applied to a scenario with LTE and 5G mobile communication system.
Here, a terminal (user equipment, UE) capable of accessing to LTE network and 5G network is connected to a 5GAP being an access point of a 5G mobile communication system and to an evolved NodeB (eNB) being an access point of a 3.9/4G mobile communication system (LTE). The control plane (dotted lines) as well as the user plane (solid lines) of both mobile communication systems is handled by a control plane mobile gateway (cMGW) and a user plane gateway (uGW), respectively, via respective interfaces (S1*, S1-U, S1-C).
3rd Generation Partnership Project (3GPP) switched from the distributed architecture of wideband code division multiple access (WCDMA) to a flat architecture of LTE which allowed a reduction of the number of hardware boxes, a reduction of intermediate nodes, and a minimization of Access Stratum (AS) signaling during mobility.
The introduction of small cells with limited coverage compared to a large macro cell results in that the frequency of mobility events involving change of small cell base stations increases. That is, AS mobility events causes excessive signaling on the network, which consumes additional processing, additional signaling, and may cause service interruption during the mobility events.
However, it is expected that 5G is to have multiple flavors of small cells in centimeter wave and millimeter wave. Accordingly, the mentioned problem resulting from small cells is likely to be considerably increased.
Hence, there is a need to reduce signaling and to minimize service disruption during mobility events.
In particular, there is a need to provide for improvements in small cell mobility with dual/multi connectivity.
In this regard it is noted that dual connectivity means that a terminal moves between two radio access coverage areas having different RATs (here LTE and 5G), establishing simultaneous connections with both networks before seamlessly handing over.
Further, multi connectivity means that a terminal connects to two base stations of a RAT (here 5G) simultaneously, improving bit rate performance through multiple downlink streams, as well as signal strength and resilience.